The present invention relates to thermoforming three dimensionally contoured articles such as automobile headliners. In the past, many automobile headliners have been formed by molding them of fiberglass reinforced polyester resin in a lay up molding process, as distinguished from thermoforming. VanDresser Inc. molds such a product of glass reinforced polyester resin, laminated to a rigid urethane foam and covered by a soft urethane backed fabric. Such headliners act as a sound board, thus making the interior of the vehicle more noisy. Further, the lay up molding process is costly.
Some have eliminated the boardiness of fiberglass resin headliners and the necessity of lay up molding by thermoforming a laminate comprised of a stiff structural yet thermoformable polystyrene foam and either kraft paper or a polymer film material bonded to either side of the foam. This laminate is covered with a soft polyurethane backed fabric. Thermoforming as opposed to lay up molding is cost effective. A laminate of a stiff polystyrene core with kraft paper or polymer film bonded to either side can be readily mass produced on automated equipment, cut into sheets, heated in a thermoforming operation and vacuum formed to shape. However, such headliners do not have sufficiently desirable sound absorbing properties because the kraft paper tends to reflect the sound rather than absorb it.
Many have endeavored to eliminate the paper or polymer film covering from such laminates and substitute a nonwoven fabric layer on one or both sides of the stiff polystyrene foam core in order to achieve better sound absorbing properties. One problem with such an approach is that automobile headliners must be able to withstand relatively high environmental use temperatures, i.e., 185 Degrees F. The structural foam polystyrene-fabric laminate tends to sag when exposed to such high temperatures.
The obvious solutions to the sagging problem include:
(1) using a higher temperature foamed polymer; and/or PA1 (2) laminating nonwoven fabric to the polystyrene foam core which incorporates polymer binders having softening temperatures in the neighborhood of or above 185 Degrees F.
One drawback to using higher heat resistant polymer is that the foam core becomes increasingly difficult to thermoform into three dimensionally contoured configurations. Also, the material tends to be more rigid and less sound deadening than the stiff thermoformable polystyrene. The use of polymer binders in a nonwoven fabric which is laminated to the foam core having softening temperatures greater than 75 Degrees C. as suggested in U.S. Pat. No. 4,529,641 to Holtrop et al. does not seem to satisfactorily eliminate the sagging problem. The line of tolerance between using materials having excessive heat resistance, making thermoforming more difficult, and those having sufficient heat resistance to pass the environmental heating tests appears to be a difficult line to walk.
The '641 patent also suggests the possibility of using a thermoset phenolic resin as the nonwoven fabric binder. The theory of such use is to heat and thermoform the laminate before the thermosetting resin cures. Once it cures, forming is no longer possible. Having once set, the thermosetting phenolic binder would be resistant to any further deformation as a result of exposure to environmental temperatures of up to 185 Degrees F.
The problems with this theory are:
(1) the thermosetting phenolic polymer may set during the heating step, and become nonformable when placed in the mold; and/or
(2) the thermoset phenolic polymer component of the laminate may exhibit an undesirably short shelf life, i.e., set during storage, shipping and the like, thereby rendering the laminate non-thermoformable.
Those skilled in this art have spent years searching unsuccessfully for solutions to these problems. As a result, thermoformed kraft paper or polymer film and structural foam polystyrene laminates and lay up molded fiberglass reinforced polyester resin based laminates remain the principal automobile headliner constructions in use today.